In recent years, there is an increasing demand for bisphenol A as a principal raw material for engineering plastics such as polycarbonates and epoxy resins. Bisphenol A is usually obtained by the reaction of excess phenol with acetone in the presence of an acidic catalyst. Cation exchange resins are most common for such catalyst and those containing sulfonic acid groups are normally used. Of cation exchange resins containing sulfonic acid groups, sulfonated styrene-divinylbenzene copolymers are put to practical use most widely.
Sulfonated styrene-divinylbenzene copolymers are resins obtained by copolymerizing styrene and divinylbenzene in the presence of a polymerization initiator and introducing sulfonic acid groups to the aromatic rings of the styrene and divinylbenzene in the resulting copolymers with the aid of sulfuric acid and the like. The copolymers in question are considered to have a complex three-dimensional network in which divinylbenzene bridges polystyrene chains to form an irregularly intertwined structure. An increase in the amount of divinylbenzene leads to increased branching of the polystyrene chain and a denser network. Conversely, a decrease in the amount of divinylbenzene leads to decreased branching and a coarser network. The divinylbenzene here plays the role of a knot in the network and is referred to as crosslinking agent. The ratio of the amount of crosslinking agent added to the amount of total monomers used is generally designated as degree of crosslinking.
The aforementioned copolymers ranging widely in the degree of crosslinking have been used as a catalyst for the preparation of bisphenol A and the existence of a significant correlation has been reported between the degree of crosslinking and the catalyst life. For example, Japan Kokai Tokkyo Koho Hei 6-32755 (1994) teaches the use of cation exchange resins with a degree of crossliking of 6 wt. % or less since the life of a cation exchange resin catalyst becomes longer as the degree of crosslinking becomes lower. However, a cation exchange resin catalyst with a lower degree of crosslinking generally contains less sulfonic acid groups per unit volume of catalyst and leads to reduced output of bisphenol A. Moreover, a cation exchange resin catalyst with a lower degree of crosslinking is more susceptible to elastic deformation when subjected to a stress from the outside and, when used in a fixed-bed reactor, may occasionally cause such pressure loss as to render the production difficult on a commercial scale. Thus, elongation of the catalyst life by reducing the degree of crosslinking causes another problem and an improved procedure is desired for development of cation exchange resin catalysts which maintain the production of bisphenol A over a long period of time, resist elastic deformation and possess high strength without decreasing the number of sulfonic acid groups per unit volume of catalyst.
An object of this invention is to provide cation exchange resin catalysts which show high productivity per unit volume of catalyst, have a long life, maintain high productivity of bisphenol A over a long period of time, resist elastic deformation and possess high strength. Another object of this invention is to provide an efficient process for preparing bisphenol A.